Electrochemical cells are energy conversion devices that include fuel cells and electrolyzer cells. Fuel cells are used to generate electrical energy using various fuels, whilst electrolyzer cells are used to generate hydrogen gas from hydrogen-containing fluids.
A typical solid polymer water electrolyzer (SPWE) or proton exchange membrane (PEM) electrolyzer includes an anode, a cathode and a PEM disposed between the two electrodes. The PEM is a selective electrolytic membrane with a catalyst layer on each side. An electrolyzer uses electricity to electrolyze water to generate oxygen from its anode and hydrogen from its cathode. Water is introduced to, for example, the anode of the electrolyzer which is connected to the positive pole of a suitable direct current voltage. Oxygen is produced at the anode by a reaction according to the equation H2O=½O2+2H++2e−. The protons then migrate from the anode to the cathode through the membrane. On the cathode, which is connected to a negative pole of the direct current voltage, the protons that are conducted through the PEM are reduced to hydrogen according to the equation 2H++2e−=H2.
A conventional PEM fuel cell includes an anode, and a cathode with the PEM disposed between the anode and cathode. A fuel cell generates electricity by bringing a fuel gas (typically hydrogen) and an oxidant gas (typically oxygen) to the anode and the cathode respectively. In reaction, the fuel is oxidized at the anode to form cations (protons) and electrons according to the equation: H2=2H++2e−.  The PEM facilitates the migration of protons from the anode to the cathode while preventing the electrons from passing therethrough. As a result, the electrons are forced to flow through an external circuit thus providing an electrical current. At the cathode, oxygen reacts with electrons returned from the electrical circuit to form anions. The anions formed at the cathode react with the protons that have crossed the membrane to form liquid water as the reaction by-product according to the equation: ½O2+2H++2e−=H2O.
More particularly, a typical fuel cell employing a PEM comprises an anode flow field plate, a cathode flow field plate, and a membrane electrode assembly (MEA) disposed between the anode and the cathode flow field plates. Each reactant flow field plate has an inlet region, an outlet region, and open-faced channels to fluidly connect the inlet to the outlet, and provide a way for distributing reactant gases to the outer surfaces of the MEA. Achieving good performance with a fuel cell requires that the reactant gases are evenly distributed over the entire surface of the active area of the respective flow field plates. This is achieved through the use of a flow control structure known as a flow field on the active surfaces of the flow field plates. The flow field plates are fabricated from conductive materials and the flow fields typically include a pattern of grooves and lands.
The MEA comprises the PEM disposed between an anode catalyst layer and a cathode catalyst layer. The PEM may be a suitable proton conducting material such as an ionomer and the like. The catalyst layers include electrocatalysts such as platinum supported on fine carbon which provides sufficient electrical conduction for electrons. The MEAs are also properly supported in the electrochemical cell when assembled to prevent the leakage of process fluids.
A first gas diffusion media (GDM) is disposed between the anode catalyst layer and the anode flow field plate, and a second GDM is disposed between the cathode catalyst layer and the cathode flow field plate. The GDMs facilitate the diffusion of the reactant gas, either the fuel or the oxidant, to the catalyst surfaces of the MEA. Furthermore, the GDMs enhance the electrical conductivity between each of the anode and cathode flow field plates and the electrodes.
Conventional fuel cells generate relatively low voltages. In order to provide a useable amount of power, fuel cells are commonly configured into fuel cell stacks, which typically may have 10, 20, 30 or even hundreds of fuel cells in a single stack. While this does provide a single unit capable of generating useful amounts of power at usable voltages, the design can be quite complex and can include numerous elements, all of which must be carefully assembled.
This basic cell structure itself requires two seals, each seal being provided between one of the flow field plates and the PEM. Moreover, these seals have to be of a relatively complex configuration. In particular, as detailed below, the flow field plates, for use in a fuel cell stack, have to provide a number of functions and a complex sealing arrangement is required.
For a fuel cell stack, the flow field plates typically provide apertures or openings at either end, so that a stack of flow field plates then define elongate channels extending perpendicularly to the flow field plates to form distribution channels extending through the entire fuel cell stack. As a fuel cell requires flows of a fuel, an oxidant and a coolant, this typically requires three pairs of ports or six ports in total for a fuel cell with three ports for each side of the flow field plates. This is because it is necessary for the fuel and the oxidant to flow through each fuel cell. However, it is possible to have multiple inlets and outlets to the fuel cell for each fluid depending on the stack/cell design. Any combination can be envisioned as long as there is a continuous flow through the fuel cell to ensure that, while most of the fuel or oxidant (as the case may be) is consumed, any contaminants are continually flushed through the fuel cell. It will thus be appreciated that the sealing requirements may be complex.
The coolant commonly flows across the back of each fuel cell, so as to flow between adjacent, individual fuel cells. This is not essential however and, as a result, many fuel cell stack designs have cooling channels only at every 2nd, 3rd or 4th (etc.) plate. This allows for a more compact stack (thinner plates) but may provide less than satisfactory cooling. This provides the requirement for another seal, namely a seal between each adjacent pair of individual fuel cells.
The foregoing assumes that the fuel cell is a compact type of configuration provided with water or the like as a coolant. There are known stack configurations, which use air as a coolant, either relying on natural convection or by forced convection. Such cell stacks typically provide open channels through the stacks for the coolant, and the sealing requirements are lessened. Commonly, it is then only necessary to provide sealed supply channels for the oxidant and the fuel.
Commonly, the seals are formed by providing channels or grooves in the flow field plates, and then providing prefabricated gaskets in these channels or grooves to effect a seal. In known manner, the gaskets (and/or seal materials) are specifically polymerized and formulated to resist degradation from contact with the various materials of construction in the fuel cell, various gasses and coolants which can be aqueous, organic and inorganic fluids used for heat transfer. Reference to a resilient seal here refers typically to a floppy gasket seal molded separately from the individual elements of the fuel cells by known methods such as injection, transfer or compression molding of elastomers. By known methods, such as insert injection molding, a resilient seal can be fabricated on a plate, and clearly assembly of the unit can then be simpler, but forming such a seal can be difficult and expensive due to inherent processing variables such as mold wear, tolerances in fabricated plates and material changes. In addition custom made tooling is required for each seal and plate design.
A fuel cell stack, after assembly, is commonly clamped to secure the elements and ensure that adequate compression is applied to the seals and active area of the fuel cell stack. This method ensures that the contact resistance is minimized and the electrical resistance of the cells is at a minimum. To this end, a fuel cell stack typically has two substantial end plates, which are configured to be sufficiently rigid so that their deflection under pressure is within acceptable tolerances. The fuel cell also typically has current bus bars to collect and concentrate the current from the fuel cell to a small pick up point and the current is then transferred to the load via conductors. Insulation plates may also be used to isolate, both thermally and electrically, the current bus bars and endplates from each other. A plurality of tension rods, bolts and the like are then provided so that the fuel cell stack can be clamped together. Rivets, straps, piano wire, metal plates and other mechanisms can also be used to clamp the stack together. To assemble the stack, the tension rods are provided extending through one of the plates, an insulator plate and then a bus bar (including seals) are placed on top of the endplate, and the individual elements of the fuel cell are then built up within the space defined by the rods or defined by some other positioning tool.
One problem with many electrochemical cell designs is the layout of the catalyst on the MEA. Commonly, the layout of the catalyst is such that the reactant gases or reactant fluids, provided by the inlet apertures in the anode or cathode flow field plates, directly impacts the catalyst when first introduced onto the MEA. At this point, the flow of the reactant gas is quite strong and turbulent and the reactant gases are at their highest concentrations in the vicinity of the inlet apertures. As a result, the catalyst in the vicinity of the inlet aperture of a given flow field plate tends to overreact with the reactant gas. The over-reaction at the catalyst in the vicinity of the inlet apertures produces a temperature increase in this area.
A similar problem occurs for the catalyst located in the vicinity of an outlet aperture for a given flow field plate. Typically, in a flow field plate, an inlet aperture feeds at least one main reactant gas flow channel which branches into a plurality of reactant gas flow channels to distribute the reactant gas flow across much of the flow field plate. The plurality of reactant gas flow channels then recombine into at least one main reactant gas channel which then feeds an outlet aperture. Since, the reactant gas flow channels are recombined near the outlet aperture, there is an increase in reactant gas flow. Accordingly, there is a tendency for the catalyst in the vicinity of the outlet aperture to over-react with the reactant gas. Similar problems of temperature increase, and gas flow turbulence result.
This additional heat may prematurely erode the membrane in those areas. In addition, the reactant gas travels through the plate in these areas, in a known backside feed manner, with the flow changing from the z plane to the x-y plane as the reactant gas flow through the inlet aperture and away from the inlet aperture to the membrane with the reverse occurring at the outlet apertures. This action may cause a lot of localized turbulence or pressure build-up, as well as higher reactant gas velocity, that may also damage the membrane. Typically the gases are coolest at the inlets and hottest at the outlets. This may also cause erosion of the membrane at the outlet apertures.
The temperature increase and the turbulent reactant gas flow also reduce the efficiency at which the fuel cell operates. Both the temperature increase and the turbulent gas flow also increase the rate of erosion of the catalyst located near the inlet and outlet apertures. Consequently, the lifetime of the MEA is reduced. This also results in an increase in the frequency at which the fuel cell stack undergoes maintenance. Further, the pressure is higher at the inlet apertures compared to the outlet apertures since some of the reactant gas diffuses across the GDM as the reactant gas flows across the face of the flow field plate from the inlet aperture to the outlet aperture. This added pressure may also affect the structural integrity of the membrane at the inlet apertures.